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Twelve-year Conifer and Vegetation Responses to Discing and Glyphosate Treatments on a BWBSmw Backlog Site

Author(s) or contact(s): B.S. Biring, W.J. Hays-Byl, and S.E. Hoyles
Source: Research Branch
Subject: Herbicides and Fungicides
Series: Working Paper
Other details:  Published 1999. Hardcopy is available.


In 1982, an operational vegetation management trial (sx82502g-5) was established to examine the effectiveness of vegetation control treatments applied to a mechanically cleared backlog site in the moist warm Boreal White and Black Spruce biogeoclimatic subzone of British Columbia. Two plots of 1.2 ha were established on the site. One of the plots was disced, using a Komatsu D65 crawler tractor equipped with a breaking disc, prior to planting Picea glauca (white spruce) in spring of 1983. The herbicide glyphosate (Roundup® @ 2.4 kg a.i./ha) was applied to one-half of the disced and undisced plots in late summer. Both discing and glyphosate application resulted in the following treatment areas: (1) discing; (2) discing + glyphosate; (3) glyphosate; and (4) untreated control. The study site was remeasured in the fall of 1995 (seedling measurements) and summer of 1996 (vegetation assessments) to determine whether treatments had any lasting effect on white spruce seedlings and on the vegetation community.

Twelve years after treatment, discing improved white spruce basal dia-meter and had no significant effect on spruce height and crown diameter. However, glyphosate treatment significantly increased basal diameter, height, and crown diameter, and reduced height-to-diameter ratio of white spruce seedlings compared with the untreated seedlings. Treatments had no significant effect on the number of well-spaced spruce seedlings per hectare. However, only plots treated with glyphosate achieved the minimum free-growing requirements 12 years after white spruce planting. The spruce seedlings in the untreated control and disced plots were not free-growing based on the current free-growing standard (which does not accept deciduous species). Significantly higher deciduous density and percent cover were found in both plots compared with the glyphosate-treated plots.

Discing and glyphosate treatments significantly reduced deciduous percent cover, density, and height by inhibiting deciduous resprouting and/or by altering species abundance compared with the untreated control. The reductions in total vegetation cover in the glyphosate-treated plots largely reflect reductions in deciduous and shrub percent cover. These reductions in percent cover in glyphosate-treated plots resulted in significant increases in plant species diversity. The glyphosate treatment significantly reduced the cover of wildlife forage species. In the absence of vegetation control, deciduous density may be sufficient to restrict wildlife movement.

Yield projections for spruce based on simulations using tipsy suggest that glyphosate application can reduce the rotation by up to 19 years compared with the untreated control. However, the glyphosate treatment also reduced deciduous timber volume by up to 46% compared with untreated areas. Since deciduous species are present as an overstorey in mixedwood stands, and should continue to occupy these stands at least to maturity, the deciduous volume should be included for total yield analysis. The deciduous volume estimated using Winvdyp in glyphosate-treated plots (< 128 m 3 /ha) was one-fourth and one-half of the volume projected in the disced (170 m 3 /ha ) and untreated control (234 m3 /ha) plots, respectively.

Working Paper 43 (842 KB)

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Updated July 24, 2015